The present invention relates to a dies set for magnetizing the outer surface of a columnar magnet adapted for use in electrostatic developing apparatus of magnetic brush developing type and, more particularly, to a dies set for imparting a plurality of axially extending poles to a cylindrical surface of a columnar magnet which is generally referred to as a magnetic roll.
Developers conventionally used for developing latent images on an electrostatic latent-image carrier such as a photoconductive body are classified into dry type developer consisting of dry powders, and wet type developer consisting of developing powders dispersed in a suitable solvent. From another point of view, the developers are classified into bicomponent developer consisting of two components of magnetic carrier and toner particles, and unicomponent developer in which magnetic particles are included in the toner particles.
Also, the developing methods, i.e., the methods for depositing the developing particles on a latent image carried by an electrostatic latent image carrier, are generally classified into cascade type and magnetic brush type. In the past, the cascade type developing method had been widely used. However, due to the so-called edge effect or fringing effect, which disadvantageously causes an insufficient developing at the central portion of the region to be developed, and due to other disadvantages inherent in the cascade type method, the magnetic brush type developing method has been getting popular in recent years.
In the developing apparatus of the magnetic brush type, the developer particles are conveyed to the region of a latent image on a carrier, in accordance with the rotation of a developing roll. The developer particles then form a protruding mass in the form of a brush, in the area close to the electrostatic latent image, by the attracting force caused by a permanent magnet incorporated in the developing roll. The developer particles are deposited on the latent image to render the latter visible, as the image is rubbed by the brush-like mass of developing particles, as a result of the rotation of the developing roll or of the movement of the image itself. The developing roll consists of a columnar permanent magnet provided with a supporting or rotary shaft, and a cylindrical shell provided to coaxially enclose the magnet for free relative rotary motion. A plurality of poles are formed on the surface of the magnet to extend in the axial direction of these magnets. It has been confirmed that the formation of the magnetic brush is greatly affected by the pattern of arrangement of these poles.
The present invention aims at providing a dies set for a magnetizing device capable of magnetizing the columnar magnets in such a manner as to impart a uniform distribution of particle-attracting force to the surface of the cylindrical shell, even though the magnet is rotating.
To this end, according to the invention, there is provided a dies set having dies made of soft iron in which is formed a through bore for receiving a cylindrical columnar permanent magnet material to be magnetized. A plurality of grooves are formed in the wall of the through bore to extend in the axial direction of the magnet. Each groove is separated from each adjacent groove by an axially extending protrusion which forms a magnetic pole of the dies. The top surfaces of these magnetic poles are therefore put in the close proximity of the peripheral surface of the magnet material. Magnetizing coils are received by the grooves in such a manner that the magnetizing currents in coils of adjacent grooves run in the opposite axial directions.
For obtaining a regular pattern repetition of circumferentially alternating magnetic poles along the surface of the cylindrical columnar permanent magnet, all of the magnetic poles protruding from the peripheral wall of the through bore of the dies set preferably have an equal circumferential breadth. Also, the grooves in the wall of the through bore should have an equal breadth.
At the same time, it is highly preferred that the grooves are fully filled with the magnetizing coils as uniformly as possible, so as to obtain a monotonical distribution of magnetic field between the adjacent magnetic poles.
Further, the breadth of the magnetic pole of the dies set is preferably selected to be 0.16 to 0.5 times that of the grooves which are separated by the magnetic pole.
Although not exclusive, the dies set is preferably divided into a plurality of segments, along plane or planes which passes the central axis of the columnar magnet to be magnetized when it is in therein, for an easier magnetizing operation.
The above and other objects, as well as advantageous features, of the invention will become clear from the following description of preferred embodiments taken in conjunction with the attached drawings in which: